A potentially interesting experiment which I'm sure could eventually lead to some highly lucrative cupboard design improvements. However I believe the empirical evidence of statistics gathered by the methods you have outlined will unfortunately be fatally flawed because of the variable kitchen parameters.
i.e.
You do not specify whether you have used Salted or Unsalted butter for lubrication. Both of which will have different frictional coefficients (despite both being from the 'butter' family) One poster above has already described using Extra Virgin Olive oil which is entirely different again. These different frictional coefficients will alter the harmonics reaching the testers ears.
Also I'm fairly sure that one or other cupboard would be nearer to either a fridge/freezer or cooker. Hence the air in the opposing cupboards would be either cooler (more dense) or hotter (less dense) which would, again, adversely affect the acoustical properties.
Likewise the positioning of the cupboards relative to doors, windows and extractor fans in the kitchens will affect the airflow around the cupboards.
You make no mention of the cupboard contents (plates, glasses, tinned goods, or even bare) all of which will affect the amount of reverberation recorded.
Hair length and ear 'jewelry' will play a part too, etc. etc.
Alas I fear that as desirable as these statistics would be, until you provide concise definitions to the address the above variations, we will never find the solution to this Holy Grail of Cupboardry.